The present invention pertains to weatherable coating systems suitable for application to thermoset or thermoplastic composite surfaces having minimal surface porosity. More particularly, the present invention pertains to a stain, a topcoat, and to a stain and topcoat system which can be easily applied by the consumer to produce an aesthetic, weatherable surface, particularly on exterior doors and door entry components.
Conventional finishing methods for exterior doors and door system components, where a traditional finely finished wood grain appearance is desired, have involved staining a bare wood surface with dye and/or pigment-bearing stains followed by application of one or more coats of exterior varnish. A seal coat is sometimes applied prior to the stain and topcoat to seal the pores of the wood and to promote a more adherent and uniform stain and topcoat. The stain is virtually always solvent borne, as water based stains have the effect of raising the grain, thus requiring an intermediate sanding step which is labor and time intensive. In addition to the solvent contained in the stain, stains contain drying oils as carriers, for example heat-bodied linseed oil. Generally, several different oils are provided to promote penetration of the stain into the wood surface.
After the stain has dried, one or more transparent topcoats are applied. Traditional topcoats are varnishes containing dissolved resins such as copal, often in conjunction with a drying oil. More recently, chemically modified natural resins and synthetic polymers have replaced all or part of natural resins, especially for outdoor use. In addition, to provide superior weatherability, ultraviolet absorbers have been added. Despite the advances made in such finishes, varnishes still generally require light sanding or abrading with steel wool or the like between coats, or necessitate the addition of the second coat prior to the full drying of the previous coat. Otherwise, delamination between varnish coats may occur. Sanding and similar treatments are time consuming, and judging the proper time to apply a second coat onto a not fully-cured prior coat requires some skill and experience. Despite these drawbacks, stain/varnish systems continue to be used on exterior door systems.
Recently, high quality exterior door systems employing molded thermoset and/or thermoplastic composite skins have become commercially available. These door systems are compression molded to exhibit a wood grain appearance, and when properly finished, are difficult to distinguish from natural wood doors. These composite doors offer several advantages over natural wood products, such as freedom from warpage, lower cost, greater insulation efficiency, and the like. Unfortunately, the stain/varnish systems in use for finishing natural wood doors are only marginally acceptable for composite doors. A satisfactory finish may often be achieved under production conditions where skilled workers, sophisticated spray equipment, drying ovens and the like are available. However, such systems are ill-suited for application by the consumer.
The difference between the ease with which finishes may be applied to natural wood products on the one hand and composite exterior door systems on the other, is due to the nature of the materials involved. Natural wood is a porous and absorbent product. Hence, dyes may penetrate the wood fibers as well as into the wood pores. The latter may also serve to trap and retain pigment, as may partially abraded fibers generated in sanding the wood prior to finishing. However, composite door skins are constructed of sheet molding compound (SMC) or other thermoset or thermoplastic matrices, often containing fillers which may range from wood flour to finely ground minerals and/or glass fibers. Despite the presence of fillers, the composite door skins are essentially non-porous. Thus, stains containing dyes as the colorant are largely ineffectual, and pigmented stains require a higher amount of pigment than would be required for a conventional wood stain. The lack of porosity and the generally smooth surface of such products require the stain pigments to be physically adhered to the surface.
Furthermore, the drying oils used in conventional stains, particularly those of lower viscosity, act as subsurface emollients in wood, but remain as an exterior thin film on composite skins. Lower molecular weight oils, being unable to penetrate the essentially non-porous surface, may remain tacky or require lengthy drying times or oven cure. During weathering due to light exposure, these oils experience scission of naturally occurring unsaturated molecular sites. The scission produces a variety of lower molecular weight, liquid and often volatile components, which causes the subsequently applied topcoat to blister, coming off in sheets.
The detrimental effects of drying oil vehicles may be minimized by substituting quickly evaporating solvents such as mineral spirits, naphtha, or the like. However, such stains tend to dry rapidly, leaving application marks, i.e., xe2x80x9cbrush marksxe2x80x9d where overlapping coats of stain are applied. Elimination of drying oil components results in little adherence of pigment to the surface. This minimal adherence may be acceptable in porous products, but is highly problematic when non-porous surfaces are involved.
Adkins, et al., in U.S. Pat. No. 4,923,760, discloses water-borne emulsions employing two different maleinized linseed oils together with cosolvents such as propylene gylcol tertiary butyl ether and a variety of hydrocarbons. However, such stains are still formulated for porous wood surfaces and are not well suited to non-porous composite door skins. Lowering the amount of the oil vehicles would be appropriate, but the stains then dry too fast and have only a limited capacity to stably disperse pigment.
Pigmented stains, whether solvent-borne or water borne, are traditionally protected by aromatic polyurethane topcoats. When the previously deposited stain contains low levels of scissionable drying oils, failure by blistering and peeling is substantially minimized. However, aromatic polyurethanes are subject to severe degradation by light. Exposure to sunlight causes degradation which ultimately results in ablative loss of the coating when applied to non-porous surfaces. Polyurethane varnishes often contain sacrificial ultraviolet (UV) protectants. However, the ultraviolet light protection package is typically consumed in as few as eight months exposure to sunlight. As the stain coat is not strongly adhered to non-porous substrates, failure of the topcoat results in ablative loss of the stain layer, typically in less than two months after the topcoat is breached.
Aqueous topcoats have been used with some success, as indicated by the disclosures of U.S. patents to Gobel et al. (U.S. Pat. No. 5,342,882) and Sanders et al. (U.S. Pat. No. 4,509,981). Water-based topcoats employing aliphatic urethane-acrylic latexes are commercially available. The use of a non-sacrificial UV protectant package in these coating compositions provides a substantial increase in durability of underlying resin-bound solid pigmented coating relative to the degree of protection provided by aromatic polyurethane finishes. However, these finishes are incompatible with pigmented stains having little or no resin binder and high pigment loading. This incompatibility is believed due to the relative differences between the somewhat hydrophilic pigment particles and the hydrophobic topcoat.
It would be desirable to provide a pigmented stain suitable for use on relatively non-porous thermoset and thermoplastic composite surfaces, particularly door skins. It would be further desirable to provide such pigmented stains in a formulation which can easily be applied by the consumer in a uniform manner, without resort to the use of expensive and not readily available production equipment. It would be yet further desirable to provide a topcoat which is compatible with the pigmented stain, and which exhibits a high, continuing degree of ultraviolet protection and weathering resistance. The stain/topcoat system should not employ two-component coatings with limited pot life, and should be removable by traditional paint strippers should the need arise.
The present invention pertains to a pigmented stain containing a unique resin binder, which is highly effective to stain thermoset and/or thermoplastic, essentially non-porous surfaces; and to an aqueous, transparent topcoat which is compatible therewith. The stain/topcoat system provides a finish system which is suitable for consumer application, which can be removed by conventional paint strippers, and which, following application, presents a highly aesthetic, weatherable surface.
The pigmented stain of the present invention contains a high loading of pigments necessary to impart the degree of coloration required for non-porous thermoset and/or thermoplastic composite materials, particularly door skins, trim, etc., and a specific resin binder, in a solvent system which preferably employs a tail vehicle in addition to conventional fast drying solvents. The resin binder is a dicyclopentadiene-modified drying oil as disclosed by U.S. Pat. No. 5,288,805, herein incorporated by reference, and commercially available as DILULIN(trademark) copolymer resins from Cargill, Inc. It has been surprisingly found that one copolymer resin is adequate not only to provide the requisite adhesion of pigment to substrate, but also, in conjunction with the formulation of the topcoat, to allow for the use of large amounts of pigment in the stain without compatibility problems. Thus, it is preferred that the dicyclopentadiene-modified drying oils constitute the largest part, and preferably all or substantially all of the resin binder.
The dicyclopentadiene-modified drying oil is preferably prepared by reacting dicyclopentadiene in an amount of from about 10 weight percent to about 40 weight percent with a drying oil or mixture of drying oils having relatively high iodine numbers, preferably above 150, and more preferably in the range of 170 to 195. Linseed oil is the preferred drying oil, however other oils such as tung oil, dehydrated castor oil, and other oils may be used as well, or in admixture with linseed oil. The dicyclopentadiene is added to the drying oil in small increments at relatively high temperature, i.e., 250-270xc2x0 C. with stirring. It is believed that the dicyclopentadiene is converted to cyclopentadiene which then reacts with the unsaturated sites of the drying oils in a Diels-Alder reaction. The preparation of such modified oils and their use with oil-modified polyurethanes in polyurethane varnish is disclosed in U.S. Pat. No. 5,288,805.
The dicyclopentadiene-modified linseed oil is present in the stain in an amount of from about 20 weight percent to about 45 weight percent, more preferably 30 to 40 weight percent, and most preferably about 32 to 38 weight percent. The pigment loading is from about 15 weight percent to about 35 weight percent, more preferably 20 weight percent to about 30 weight percent, and most preferably about 25 to 30 weight percent. Solvents are present in amounts of about 20 weight percent to 40 weight percent, more preferably 25 to 35 weight percent, and most preferably about 30-35 weight percent. Of the total solvents, approximately 10 weight percent to about 30 weight percent, preferably 15 weight percent to about 25 weight percent are slowly evaporating solvents commonly known as xe2x80x9ctail solvents.xe2x80x9d The remainder of the solvents exhibit fast to moderate evaporation, such as mineral spirits, naphtha, solvent 142 petroleum distillate, and the like. It is preferred to use combinations of these faster evaporating solvents such that the solvents flash off at different intervals. The majority of solvent, i.e.,  greater than 50%, should flash off within a period of from 2 to 10 minutes. Most preferably, solvent 142 petroleum distillate, mineral spirits, and tridecyl alcohol tail solvent are employed in a weight ratio of about 10:10:5, with the weight ratio of dicyclopentadiene-modified linseed oil to tail solvent being approximately 3:1.
In addition to pigments, dicyclopentadiene-modified linseed oil, and solvents, the stain may contain other known additives, for example, leveling agents; drying agents, i.e., metal naphthenates such as cobalt naphthenate, calcium naphthenate, and magnesium naphthenate; ultraviolet absorbers, preferably of the non-sacrificial type, i.e., hindered amine stabilizers such as TINUVIN(copyright) 292 and TINUVIN(copyright) 328; wax; thickeners such as various clay minerals; and the like. The stain is preferably free of ordinary drying oils such as unmodified or bodied linseed oil, tung oil, and the like; and is preferably also free of other secondary resins such as urethane resins and oil-modified alkyd polymers.
The preparation of the stain is not overly critical, and may be performed by conventional paint preparation techniques. For example, the pigments, in standard commercial grade, are added to a first portion of the dicyclopentadiene-modified linseed oil and well blended in a mixing tank, three roll mill, Cowles mixer, etc. Sand milling or ball milling may also be used. In some cases, pigment size reduction is desired in addition to full wetting of the pigment-surface. A surfactant such as BYK(trademark) 156 may aid in dispersing and wetting the pigment with the resin binder. Following wetting of the pigment and any desired size reduction, the resulting mill base is diluted with solvent, additional resin added, and various driers, leveling agents, thickeners, and the like are added. Blending of the mixture is continued until a uniform dispersion is obtained.
The stain may be applied to the non-porous thermoset and/or thermoplastic composite by conventional means, i.e., by brushing, spraying, sponging, rolling, wiping, and the like. In general, excess stain is removed by wiping with a clean lint free rag or the like, but preferably by means of a china bristle brush. The stain is generally applied at temperatures between 5xc2x0 C. and 45xc2x0 C., and should be allowed to dry for approximately 48 hours at room temperature under dry conditions. Drying time may be adjusted depending upon the temperature and humidity. After the stain is dry, the topcoat may be applied.
The topcoat exhibits unexpectedly excellent compatibility with the inventive stain layer, particularly so in view of the fact that the topcoat is an aqueous composition. The topcoat comprises a non-siloxane, film-forming polymer latex, preferably an acrylic latex; an aliphatic or aromatic polyurethane latex, most preferably an aliphatic polyurethane latex; an acrylic vinyl latex; an acrylic polyurethane latex; an acrylic polyester latex; mixtures of the above, and the like. Most preferably FLEXTHANE(copyright) urethane-modified acrylate resins available from Air Products are used. The pH of the aqueous latexes must be suitable for the dispersed phase, and is preferably between 8 and 11, adjusted by addition of suitable acid or alkaline additives, particularly with ammonium hydroxide. FLEXTHANE(copyright) 620 is particularly preferred as the film forming polymer.
The term xe2x80x9clatexxe2x80x9d as used herein pertains to an emulsion, microemulsion, or dispersion of the respective polymer as a discontinuous phase in water as a continuous phase. The particle size is not overly critical, and may vary, for example, from smaller than 0.001 xcexcm to 2 xcexcm or larger, preferably from 0.01 xcexcm to 0.5 xcexcm. The dispersed phase will preferably remain stably dispersed in the continuous phase. If the dispersed phase settles or separates, it should be readily redispersible by means of simple agitation, as by stirring or the use of a paint mixer or the like. Latexes with small particle sizes are preferred. By the term xe2x80x9cdispersiblexe2x80x9d as it applies to the non-siloxane, film-forming polymer, is meant that the polymer may be dispersed or emulsified in water to form a latex as that term is used herein.
The topcoat composition may contain conventional additives such as cosolvents, leveling agents, flatting agents, emulsifiers and emulsion stabilizers, biocides, thickeners, suspending agents, and in particular, ultraviolet protection additives. Examples of suitable cosolvents include the various glycol ethers and acetates available from the Dow Chemical Company under the tradename DOWANOL(copyright) glycol ethers and acetates. Aliphatic alcohol cosolvents such as methanol, ethanol, n-propanol, 2-propanol, n-butanol, 2-ethylhexanol, tridecyl alcohol, and the like may be used, as can also ketones such as methylethylketone, 2-pentanone, cyclohexanone; and aliphatic and aliphatic hydrocarbons. The latter may be used in quantities such that stable emulsions may be maintained.
The key to the compatibility and weatherability of the topcoat, however, is due to the presence of an organopolysiloxane microemulsion, preferably one containing one or more curable or crosslinkable organopolysiloxanes. The presence of this microemulsion is believed to alter the hydrophobicity of the topcoat, rendering the latter compatible with the stain formulation of the present invention. Alkoxyalkyl-functional organopolysiloxanes such as Wacker Silicones 43A organopolysiloxane are suitable, for example. Although siloxane microemulsions are preferred for use herein, stable emulsions containing larger particle sizes are useful as well.
The organopolysiloxanes are curable organopolysiloxanes which are readily available. Such curable organopolysiloxanes have reactive functional groups such as alkoxy, silanol, chloro, hydrido, acetoxy, and the like. The functional groups may be terminal groups or may be distributed along the polysiloxane chain. Preferably, the average functionality of the organopolysiloxanes is greater than two such that a crosslinked thermoset polymer is obtained upon cure. However, lower functionality organopolysiloxanes are also suitable, so long as the improved topcoat weatherability is maintained. Suitable organopolysiloxanes are identified in U.S. Pat. No. 4,913,972, which is herein incorporated by reference. A preferred organopolysiloxane is Wacker 43A organopolysiloxane, available from Wacker Silicones, Adrian, Mich. organopolysiloxanes which contain unsaturated organic groups such as allyl-, vinyl-, vinylether, and other unsaturation-functional groups are also useful. These organopolysiloxanes may further crosslink under the influence of light exposure.
The term xe2x80x9ccurablexe2x80x9d includes resins which are crosslinkable as well. The term xe2x80x9ccurablexe2x80x9d is intended to reflect a curing reaction which creates a higher molecular weight cured polymer as compared to the molecular weight of the uncured organopolysiloxane. By the term xe2x80x9cmolecular weightxe2x80x9d is meant number average molecular weight.
In addition to the curable organopolysiloxane, the topcoat may contain other organopolysiloxanes which promote water beading. These organopolysiloxanes may be curable, may be essentially inert, or may interact with other system components such as the film-forming polymer either by chemical reaction or polar or ionic attraction. Examples of water beading agents are trimethylsiloxy end-capped polydimethysiloxanes, xcex1,xcfx89-dihydroxypolydimethylsiloxanes, and in particular, amino-functional organopolysiloxanes, for example xcex1,xcfx89-[3-aminopropyl]polydimethylsiloxanes. A preferred beading agent is Wacker Silicones BS 1306 organopolysiloxane.
The topcoat preferably contains in weight percent from about 35 to 60 percent water, preferably 40-55 percent water, and more preferably about 45-55 percent water; from about 25 to about 55 percent film forming polymer, preferably from about 35 to about 50 percent, and more preferably about 35 to about 45 percent; from about 2 to about 25 percent organopolysiloxane, preferably from about 3 to 10 percent, and more preferably from about 3 to about 8 percent; and sufficient surfactant to provide a stable dispersion, preferably about 1 to about 8 percent, preferably about 1 to about 5 percent, and most preferably about 2 to about 4 percent. The surfactants may be anionic, cationic, amphoteric, or non-ionic. Mixtures of various surfactants may be used. The type of surfactant should be tailored to the type of polymer latex. For example, anionic polymer latexes may be adversely affected by the use of large quantities of cationic surfactants, as coagulation may occur. The selection of suitable surfactants is within the skill of the art once the general formulation is known.
In addition to the above ingredients, the topcoat preferably contains an ultraviolet protection package. This package may contain one or more ultraviolet absorbers, preferably of the non-sacrificial type. Finely divided inorganic particles in the nanosize range, particularly those which are colorless (white) may be used as an ultraviolet absorber. Examples are titanium dioxide, silica, and various ceramic materials. The fine particle size is generally provided by flame pyrolysis or spray pyrolysis. Such UV absorbing particles may be used in amounts up to about 5 percent by weight, preferably from about 0.1 weight percent to about 2 weight percent, and more preferably about 0.2 weight percent to about 1 weight percent.
In addition to finely divided inorganic particle UV absorbers, soluble or dispersible organic absorbers such as hindered amines, cinnamic acid esters, and the like may be used, generally in amounts up to about 5 weight percent, more preferably 0.1 to about 3 weight percent, and most preferably from about 0.5 to about 2 weight percent. Preferred UV absorbers are the various TINUVIN absorbers available from Ciba Geigy.
Without wishing to be bound to any particular theory, the inventive topcoat of the subject invention is believed to comprise an interpenetrating polymer network. Thus, it is believed that the polyurethane polymer and polysiloxane polymer cure in such a fashion that the polymers form a complex network which assists in producing a weatherable coating. This conclusion is based in part on the drying/curing behavior whereby the topcoat clears as it dries, indicating little phase separation; and also upon the fact that if siloxane is added too rapidly during topcoat preparation, soft balls of gelatinous polymer may form. It is generally known in the wood finishing arts that polysiloxanes should be avoided during finishing operations to avoid surface defects such as xe2x80x9corange peelxe2x80x9d and xe2x80x9ccats eyes.xe2x80x9d The fact that relatively large amounts of polysiloxane remain compatible with the film forming polymer may be due to the formation of an interpenetrating network which than effectively prevents phase separation.
Interpenetrating polymer networks have been disclosed as antithrombogenic polymers in U.S. Pat. Nos. 4,872,867 and 4,6616,064. However, the IPNs of the former are prepared in strong aprotic solvents, while the IPNs of the latter were prepared in the melt. Neither of these techniques can be used for transparent topcoats. Further solvent-borne IPNs prepared from urethane modified acrylates and both siliconized and non-siliconized epoxy resins are presented in xe2x80x9cCure Behavior of Silicone-Epoxies and Urethane Modified Acrylates in Interpenetrating Networksxe2x80x9d, J. COATINGS TECH. Vol. 68, No. 854, March 1996, pp 65-72.
The stain/topcoat system of the present invention displays excellent weatherability, as measured by ASTM G53-88 accelerated weathering test. Moreover, the topcoat itself, even when used over conventional oil-based stains, also displays excellent weatherability. The weatherability should exceed 1500 hours minimally by the above test, should preferably exceed 2000 hours, more preferably 2500 hours, and most preferably exceeds 3000 hours.
The preparation of the aqueous topcoat may be performed in numerous ways. However, the addition of the polysiloxane compound must be accomplished in such a manner so as to prepare a stable microemulsion. Simply adding the entire amount of siloxane at one time is generally ineffectual. It has been found advantageous to prepare a mill base containing the film forming polymer and other non-siloxane additives and utilize water or a portion of the mill base to form a polysiloxane xe2x80x9cpreblend.xe2x80x9d The preblend is then added to the remaining portion of the mill base in slow continuous or incremental additions with modest agitation such that the total addition takes place over a period of 25 minutes to 10 hours, preferably 40 minutes to 6 hours. When incremental addition is utilized, it has been found advantageous to add up to about 8 percent by weight of the preblend, more preferably 0.5 to 2.5 weight percent of the preblend at each increment, the increments spaced approximately 1-5 minutes apart, more preferably a minimum of 2 minutes, and most preferably 3 minutes apart. Thus, with 2.5 weight percent increments spaced 3 minutes apart, the entire addition of preblend to mill base will take approximately 2 hours. The preblend preferably consists of 20-80 weight percent polysiloxane and 80-20 weight percent water and/or mill base, more preferably 30-65% polysiloxane, and most preferably about 40-60% polysiloxane, the remainder preferably being mill base and/or water. The preblends advantageously contain Wacker Silicone 43A alkoxyalkyl polysiloxane or other alkoxy-functional polysiloxane in an amount of from 50 to 100% based on total polysiloxane, and 0-50% of an aminoalkyl-functional polysiloxane, preferably 0-20%, and most preferably 5-15% based on total polysiloxane. Other polysiloxanes may be used as well, for example, unsaturated photocurable or heat-curable polysiloxanes.
The topcoat is preferably applied as two layers with a total dry thickness of 10-100 xcexcm, preferably 13-75 xcexcm, and more preferably 30-60 xcexcm. The topcoat is preferably applied at ambient temperatures in the 5xc2x0 C. to 45xc2x0 C. range, with three hours between coats. While the topcoat is preferably applied over a pigmented stain on a non-porous thermoset and/or thermoplastic surface, the inventive topcoat may be applied to other surfaces devoid of stain, and to porous surfaces, stained or unstained, as well.